Calculation method for power transformer clamping rings made from laminated insulating materials

In core-form oil-filled power transformers, clamping rings are used to hold a clamping force on the windings. In this paper, data is presented from mechanical tests conducted on complete rings, using a custom-built test fixture, specifically designed to simulate the forces and conditions seen on clamping rings in power transformers. The tests were conducted on laminated wood and laminated pressboard rings, in sizes typically seen in medium power transformers. Mechanical equations that most closely apply to a ring under these loading conditions were determined, and flexural strength, modulus of elasticity and shear strength calculations were performed. A new method for calculating clamping ring thickness was determined, based on the test data. For laminated pressboard, the new calculation method was compared to a previous calculation method. A statistical interpretation of the values obtained using the new calculation method vs. the test results is presented.